(a) Field of the Invention
The present invention relates to a drive system for heavy machinery having a creeping traction unit. More particularly, it relates to improved links which are joined together by connecting pins to form an endless tread belt for the drive system, and to an improved cooperating tumbler which drives the belt.
(b) Description of the Art
Tread belt drives useful for propelling large machinery and other heavy mobile equipment which are forerunners for the present invention are shown in U.S. Pat. Nos. 2,530,379, 2,727,794, 3,680,928, and 4,278,301. In these patents, a drive system is described in which the number of links which are driven by the teeth of the drive tumbler at any given moment of time is less than the number of links which are wrapped around the tumbler. This type of system has the advantage of providing for more even wear of the links and is achieved by having the pitch angle between the tumbler teeth larger than the pitch angle of the links.
The pitch angle of the tumbler teeth is the angle at the center of the tumbler which is subtended by a line drawn from the midpoint of one tooth to the midpoint of an adjacent tooth. The pitch angle of the links is the angle at the center of the tumbler that is subtended by a line drawn from the center of the pivot pin connection of one link to the pivot pin connection of an adjacent link. A detailed discussion of pitch angles and their measurement appears in U.S. Pat. No. 2,727,794.
The described prior art type of tread belt drive system provides a practical and effective way of propelling large excavating equipment. However, the use of such tread belt drive systems is not without problems. For example, heavy machinery having tread belts are often used at excavation and construction sites where they may be exposed to extremely dirty and rocky areas. Because tread belts are typically open at the side, dirt and stones may enter and pack between the links and the drive wheel. This may cause the tension on the belt to increase, which in some cases may damage the drive motor, the drive gear box, the tumbler wheel drive teeth, the connections between links, and/or cause jamming of the belt. While some prior art systems have self-cleaning features which are useful in lowering the likelihood of this occurring, it is desirable to improve the self-cleaning characteristics of prior art tread belt drive systems still further.
Other problems which can occur in some prior art systems include breakage of the links or the teeth of the drive system when a link which is being driven by a tumbler tooth is forced radially outward of the tumbler wheel in response to the driving forces. When this happens, there can be a loss of proper driving engagement between the tumbler tooth and the link, and as the link falls back towards the tumbler, the link may improperly strike a tumbler tooth and cause a fracture of either one or both of the members making repairs necessary.
Also, due to the wear of the bearing surfaces of the tumbler and the links, the pitch angle of the links may tend to increase, and the pitch angle of the tumbler may tend to decrease. When the pitch angle of the links exceeds the tumbler pitch angle, the necessary driving relationship of the tumbler to the links is lost, and the links and/or tumbler must be rebuilt or replaced. Still another problem is the fact that many prior art systems are noisy and rough in operation.
In addition to these factors, because of the rising cost of the raw materials and other inputs which are used in producing heavy machinery, the production cost of many of these machines has increased considerably. To lower the cost of production, it is desirable to have a link which can be easily cast without the need for expensive machining, and which has a symmetry line such that if the casting is slightly irregular, and the belt when assembled is slightly out of line, rather than wasting several links, one or more links can be rotated 180 degrees to correct the alignment.
Therefore, it can be seen that the need exists for a tread belt drive system which improves upon the performance of prior systems in dealing with these problems.